The transmission of power is fundamental to the operation of a broad range of machines and is accomplished by various means, selection being driven for instance by cost or complexity. For example, in some aircraft gas turbine engines, mechanical actuation is required of variable geometry compressor stator systems or variable exhaust nozzle (VEN) systems for those equipped with augmentors. Due to the desirability of minimizing weight and size and maximizing reliability, a typical power transmission arrangement entails regulating the flow of a pressurized working fluid, such as fuel or oil, to a mechanical actuator causing the extension or retraction of a translating shaft. The shaft in turn is connected to a synchronization ring which mechanically drives the variable system to the desired geometry, through a plurality of cams, rollers, and linkages. Such structure is shown, for example, in U.S. Pat. No. 4,425,787 issued to Freid and assigned to the same assignee as the present invention.
It is well known in the art that the control of variable geometry systems in gas turbine engines is fundamental to the proper performance of the engine. Malfunctioning systems can result in among other things degradation of thrust or compressor stalls.
One component of the VEN system which must operate reliably in this particularly hostile environment is the fluidic actuator. Leakage of the working fluid past the seal apparatus which circumscribes the translating shaft, at a minimum, requires maintenance action to maintain a sufficient supply of fluid in the system to keep it operational. Excessive leakage can render a properly filled system inoperable after a short period of time and requires replacement of the actuator. A representative seal apparatus is shown in U.S. Pat. No. 3,630,531 issued to Bondi and assigned to the same assignee as the present invention. The seal is comprised of a resilient member with a recumbent "U" shaped cross section. The radially outer lip of the "U" is biased radially outwardly to form a static seal with the actuator housing bore and the radially inner lip is biased radially inwardly to form a dynamic seal with the translating shaft. The effectiveness of the seal depends among other things on the dimensional size and tolerance of the shaft and housing bore.
A typical VEN, such as that described in the aforementioned Freid patent, is comprised of a plurality of interleaved flap and seal members which cooperate to form a sealed nozzle for all actuator positions. During engine operation, the flaps and seals rub against each other both when the geometry of the nozzle is being changed, as well as when the geometry is fixed, due to aerodynamic buffeting. Due to the relatively high ambient temperature in the VEN cavity, the translatable shaft becomes coated with a sticky, viscous working fluid residue. As the cooperating flaps and seals wear, the resultant fine (abrasive metallic particles) is deposited on the sticky, extended surface of the actuator's shaft. When the shaft is retracted, the particles are drawn into the seal apparatus where it becomes trapped between the shaft and the resilient member. As the actuator cycles, the contamination becomes more severe and the shaft becomes scored along its working length affecting the ability of the resilient member to deform in sealing relationship with the shaft. At intermediate actuator positions, which correspond for example to flight idle or intermediate rated power, where the engine is operated for significant periods of time, localized scoring of the shaft is accelerated. This is because of dithering of the actuator by the control system which is attempting to maintain a fixed VEN geometry against the aerodynamic buffeting forces. What results is a progressive necking of the shaft and therefore increasing leakage at those actuator shaft locations as the clearance between the shaft and resilient member increases beyond the ability of the resilient member to deform in sealing relationship with the shaft.
As is well known in the art, a scraper is often incorporated to remove debris from actuator shafts which operate in dirty environments. It is either formed as part of the resilient member itself or as a separate element disposed between the resilient member and the source of contamination. If a separate element is used, the scraper tends to be of greater hardness than the resilient member and in some cases is metallic; however, the scraper itself must not score the shaft. Contaminants scraped off the shaft eventually collect on the face of the scraper and are forced radially outwardly. As the amount of contamination increases, the particulates are forced around the outer diameter of the scraper and once again build up against the resilient member. While delaying the onset of leakage by reducing the rate of passage of contaminants, the scraper additionally is subject to wear. Eventually, contamination of the resilient member and scoring of the shaft cause the seal apparatus to fail.